1. Field of the Invention
Aspects of the present invention relate to a method and apparatus for controlling a focusing operation, and more particularly to a method and apparatus for controlling a focusing operation between data layers of a high density optical disk having a plurality of data layers using an objective lens having a high numerical aperture and a light source with a short wavelength.
2. Description of the Related Art
In general, information storage media are employed as information recording media of an optical pickup device to record and/or reproduce information to and/or from the information storage media without contacting the information storage media. Optical disks corresponding to one type of information storage media are classified as Compact Disks (CDs) or Digital Versatile Disks (DVDs) according to an amount of information recorded, and further classified into High Density (HD)-DVDs or Blu-ray Disks (BDs), which can each record more than 15 GB.
Information storage media are continuously being developed to gradually increase recording amount capabilities. A representative method of increasing the recording amount is a method of using a recording light source having a short beam wavelength, and an objective lens having a high numerical aperture.
FIG. 1 is a cross-sectional view of a multi-layer disk. As illustrated in FIG. 1, the disk has a plurality of data layers L0, L1, L2, to Ln, wherein Ls denotes a spacer layer between data layers. The optical beam passes through a cover layer and is selectively focused on the layers Ln, L2, L1, and L0. Layer Ln is closest to the optical beam and the cover layer, and layer L0 is farthest from the optical beam and is on a substrate.
As the wavelength of a beam irradiated from a light source decreases and the numerical aperture number of an objective lens increases, the spherical aberration increases according to the variation of thickness of a transparent cover layer, resulting in degradation of a recording or reproduction signal. The amount of spherical aberration is represented using a several-order equation. If the numerical aperture of an objective lens is greater than 0.8, a high order aberration cannot be ignored, and a fourth-order spherical aberration must be considered as represented by Equation 1.
                              W          40                =                                                            n                2                            -              1                                      8              ⁢                              n                3                                              ⁢                      NA            4                    ⁢          Δ          ⁢                                          ⁢          d                                    (        1        )            Here, n denotes an index of refraction of a transparent cover layer, NA denotes a numerical aperture of an objective lens, Δd denotes a thickness variation of the transparent cover layer, and W40 denotes a fourth-order spherical aberration. According to Equation 1, in a multi-layer optical disk recording and/or reproduction apparatus using an objective lens having a high numerical aperture (e.g. an objective lens whose numerical aperture is greater than 0.8), a high-order spherical aberration is large.
In general, when a disk is loaded, a disk recording and/or reproduction apparatus determines the type of the loaded disk, performs a focusing lead-in operation for a data layer to be recorded or reproduced, and reads control information such as focus servo information, tracking servo information, and address information from the loaded disk. The focusing lead-in operation is an operation to focus a beam spot on a data layer of a disk, and focusing is performed by using the focusing lead-in operation. After spherical aberration compensation is performed to optimize an address signal or a reproduction signal, information is recorded or reproduced.
If the focus servo slips off a focusing point due to an external shock or some other force while data is being recorded to or being reproduced from an optical disk, the focus servo should quickly return to the data layer at which the focus servo was focused on before the shock occurred. When an optical disk having more than three data layers is used (as shown in FIG. 1), focusing on a target data layer is especially important.
When the lead-in operation of the focus servo is controlled for a disk having a plurality of data layers, a method of focusing a target data layer in response to a level change of a Radio Frequency Data Collection (RFDC) signal and a focus error signal FES output from an optical pickup has been suggested. Japanese Patent Publication No. JP 2006/155792 discloses a focusing method of an optical disk having two data layers L0 and L1, and is described with reference to FIGS. 2A and 2B.
FIG. 2A illustrates a method of controlling a focusing operation in an optical disk device according to the prior art. First, a method of focusing on the data layer L1, which is closer to a cover layer than the other layer L0 is to the cover layer, is described with reference to FIG. 2A. When a focusing control operation is performed for the data layer L1 which is closer to the cover layer, an optical disk recording and/or reproduction apparatus performs spherical aberration compensation for the target data layer L1 and detects an RFDC signal and a focus error signal FES illustrated in FIG. 2A while moving an objective lens upwards towards a loaded optical disk.
If the RFDC signal is higher than a slice level Srec, and if the focus error signal FES meets a zero crossing point, the optical disk recording and/or reproduction apparatus determines that the focus of an optical beam is approaching the target data layer L1 and outputs a focus-servo-on signal. Specifically, if the focus error signal FES crosses the zero crossing point (at the x-axis in FIG. 2A) when dropping from a level higher than a slice level Sfe+ to a level lower than a slice level Sfe− or vise versa, the optical disk recording and/or reproduction apparatus determines that the target data layer L1 has been identified.
Next, a method of focusing on the data layer L0 farther from the cover layer is described with reference to FIG. 2B. When a focusing control operation is performed for the data layer L0 farther from the cover layer, the optical disk recording and/or reproduction apparatus performs spherical aberration compensation for the target data layer L0 and detects an RFDC signal and a focus error signal FES illustrated in FIG. 2B while moving the objective lens towards the optical disk. The optical disk recording and/or reproduction apparatus detects the target data layer L0 as described above with reference to the target data layer L1, and waits for a redetection time Tr from a zero crossing point of the focus error signal FES. If a new data layer is not detected during the redetection time Tr, the optical disk recording and/or reproduction apparatus determines that the most recently detected data layer is the target data layer L0. In this case, since the objective lens is moving upwards, the optical disk recording and/or reproduction apparatus performs the focusing lead-in operation on the target data layer L0 by moving the objective lens back downwards for an over-run time Tover, which is an amount of time that the objective lens has been moving upward away from the target data layer L0, from when the RFDC signal becomes lower than the slice level Srec. On the other hand, if a new data layer is detected during the redetection time Tr, the optical disk recording and/or reproduction apparatus ignores the previously detected data layer L1 as not being the target data layer L0, and determines that the new data layer is the target data layer L0.
FIG. 3 is a diagram illustrating a known operation to focus an optical beam on a position in a focusing lead-in process. It is assumed in FIG. 3 that a data layer closer to a cover layer is a data layer L1, and a data layer farther from the cover layer is a data layer L0. If a target data layer is the data layer L1, when a focus error signal FES passes a zero crossing point in the data layer L1, the optical disk recording and/or reproduction apparatus determines that the focal point of an optical beam is approaching the target data layer L1 and outputs a focus-servo-on signal. If the target data layer is the data layer L0, the optical disk recording and/or reproduction apparatus detects the data layer L0 and waits for the redetection time Tr from a zero crossing point of the focus error signal FES. If a new data layer is not detected during the redetection time Tr, the optical disk recording and/or reproduction apparatus moves the objective lens downwards to move the focal point of the optical beam back on the data layer L0.
As described above, according to the prior art, a data layer is determined using the redetection time Tr. However, it is difficult to accurately measure a zero crossing point due to spherical aberration in an optical disk having multiple data layers. Thus, it is difficult to accurately measure the redetection time Tr.
In addition, in an optical disk having multiple data layers, since focusing can be performed only for a data layer that is farthest from a cover layer and a data layer that is closest to the cover layer, data layers located between the farthest data layer and the closest data layer cannot be detected.